In the present study, the interferon system was evaluated in fibroblasts from persons predisposed to leukemia or other cancers, in fibroblasts from persons with neoplastic disease, and in human tumor cells. Of 31 normal fibroblast strains from patients with tumors or diseases associated with a high incidence of malignancy, only one cell strain had a poor response to either of the two interferon inducers used, polyinosinic-polycytidylic acid and Chikungunya virus. On the other hand, cell cultures of five human tumors were much less sensitive to the antiviral effect of these interferon inducers and of human interferon and produced less interferon in response to Chikungunya virus than any of the nontumor tissues studied.